Methods and systems for laser processing a workpiece and methods and apparatus for controlling beam quality therein

ABSTRACT

Laser processing methods, systems and apparatus having a super-modulating power supply ( 6 ) or pumping subsystem ( 5 ) and high beam quality (i.e., brightness) are disclosed. The methods, systems and apparatus have significant benefits, improved operation characteristics and material processing capability over currently available methods, systems and apparatus. In at least one embodiment, the beam quality of a high power solid state laser ( 2 ) is improved in the presence of thermal lensing. High power laser cutting, scribing, and welding results are improved with a combination of modulation and high beam quality while providing for improved processing speeds.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/358,058, entitled “Method and System for Laser Welding”, filed 19Feb. 2002. This application also claims the benefit of European PatentApplications EP 02 251446.7, entitled “Laser Apparatus” filed 1 Mar.2002 and EP 02 251444.2, entitled “Laser Apparatus”, filed 1 Mar. 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods and systems for laser processing aworkpiece and methods and apparatus for controlling beam qualitytherein. An improved laser source has particular applicability to laserwelding, cutting, scribing, and similar applications.

2. Background Art

Continuous wave lasers (CW lasers) are commonly used in materialprocessing applications. These are typically solid state lasers and mayhave Nd:YAG laser rods (laser media) or other types of laser rods. Theyare commonly used for material processing operations, such as drilling,welding, cutting, ablation, heat treatment and so on, on both metal andnon-metal target materials.

Referring to FIG. 1, a CW laser depicted therein comprises a laserresonator 1 including at least one laser rod 2 (typically an Nd:YAGelement) which is mounted between two flat mirrors 3 and 4. The mirror 3is a high reflectivity rear mirror 3 and the mirror 4 is a partiallytransmissive front mirror 4, known as the output coupler. The laser rod2 is pumped by one or more pumping elements such as lamps 5, which arepowered by an electrical source 6 (e.g. AC source) that generallyincludes a resonant circuit. Typically the source 6 is designed toproduce an output of about 15 kilowatts average power and 30 kilowattspeak power.

CW lasers have a rated average power and this is shown in FIG. 2 aslevel “CW.” This level may be, for example, 1000 watts, as shown in FIG.2. Such a CW laser is generally modulated by altering the power suppliedto one of more pumping lamps to a level up to the CW level dependingupon the power required at any time during a material processingoperation. As shown in FIG. 2, the level may be dynamically varied up tothe CW level to control a processing operation. Alternatively, a DCpower supply may be used, but such use should be carefully considered asfailure rates may increase. The CW output is one that can be maintainedfor 100% of the time as an average level and the laser power ismodulated up to this. A parameter known as “process speed” is, in manycases, determined by the output power and beam quality at any time.

It is sometimes desirable to try to achieve as fast a processing time aspossible and, up to now, this has been done by using either high levelsof modulation or high beam quality. High modulation is generallyachieved with very high frequencies of well above 1500 Hz using effectslike Q-switching, phase-locking, acousto-optic modulation and so on.These effects, which are done at high modulation frequency, are designedto produce high single peak but lower average power laser outputs. Forexample, a high power q-switched laser may have high peak power atrepetition frequencies of a few KHz, but average power orders ofmagnitude less. The pulse width may be substantially less than 1microsecond resulting in energy too low for high power processing.Conversely, at very high repetition rates the average power increases,but with an accompanying decrease in peak power, stability, and pulseenergy.

The teaching of present laser technology, particularly high power CWlaser technology, is to try to improve beam quality as much as possible.Beam quality is defined by the size of a spot (or a diameter of a waistin a laser beam) and is generally measured in mm-mrad. Beam quality of alaser beam is determined by the full angle divergence of the beam timesthe diameter of the beam at a waist position (all references hereinrefer to the diameter and full-angle divergence criteria). The lower thenumerical value of the beam quality, the better for most applications,although there is generally a trade-off between beam quality and outputpower. In various CW systems, processing speeds are based on beamquality improvements. For example, excellent beam quality has beenobtained in diode pumped CW systems having limited output power.

It is well known that the worst-case beam quality of a laser isinversely proportional to the effective length of the resonator, andproportional to the area of the limiting aperture inside the resonator,which is usually the rod diameter. The best beam quality (i.e., thelowest value) is therefore derived from long resonators with smallradius limiting apertures. Although some resonators have single rods asshown in FIG. 1, often resonators have a plurality of rods arranged inseries.

Various methods are known for controlling beam quality of high powersolid state lasers having one or more rods. Several methods aredescribed in “LIA Handbook of Material Processing” pp. 42-44, 2001. Forexample, in lasers with single rods, the beam quality can be varied bymoving the output coupler and rear mirror either further apart or closertogether to improve or degrade beam quality, respectively. However, inperiodic resonators with more than one rod, this is not possible. Insuch cases, the pumping chambers have to move to maintain periodicityand symmetry. This is usually impractical due to water and powerconnections.

An alternative method of controlling beam quality is to vary the radius,and therefore area, of the limiting aperture in the resonator. In mostindustrial lasers, as described, the beam quality-defining aperture isthe rod. The beam quality can then be controlled by using differentdiameter rods. Small diameter rods can be used for high beam quality andlarger diameter rods for poorer beam quality.

Typically, improved beam quality is at the expense of lower output powerand therefore a balance has to be made. However, a limitation of thistechnique is that to obtain very good beam quality, a very smalldiameter rod is required if the pitch of the resonator is to be limitedto practical values. To obtain high power, long rods are required. Longthin rods, however, have poor stiffness and can be flexed easily bycoolant flow causing instability in the laser output. Such rods, beingvery long and very thin, are subject to breakage when mechanicallystressed in handling.

A further alternative of controlling beam quality is to place aperturesmidway between the rods in a periodic resonator or to place them closeto the output coupler or rear mirror. It is known that waists are formedin periodic resonators, and by placing an aperture at a waist position,beam quality can be controlled. One limitation of this particulartechnique is that the waist diameter, and therefore aperturetransmission, depends on the value of thermal lensing in the laser rods.At low pump power, and therefore low thermal lensing, the diameter ofthe waist is at its largest and this decreases as pump power increases.

This impacts the rise time characteristics when the laser is switchedfrom cold (or lower power) to high power. The apertures have lowertransmission until the rods reach thermal equilibrium thus limiting therise time of the laser output to about one second, for example. Manywelding and cutting processes require a laser rise time much faster thanthis, typically of 1 to 10 ms. This precludes the use of such ‘farfield’ apertures. A further limitation is that such apertures have to becentered with respect to the axis of the laser rods and also have to becooled. This adds cost and complexity to the laser design.

Hence, an improved laser apparatus for use in high power system wouldprovide beam quality management in a manner that is less sensitive tothermal variations in a particular laser arrangement (e.g. single rodand periodic resonators). Such an improvement could be applied to pulsedand CW systems.

Another current tenent of laser technology presumes that modulating abeam at low levels would only present an advantage with low beam qualityequipment (i.e. numerical values significantly greater that 100 mmmrad). It has therefore always been assumed, up to now, that either highbeam quality, or modulation is required.

While present laser apparatus work reasonably well, there is always roomfor improvement and efficiency in terms of cost and better materialprocessing and other uses of laser apparatus. The present inventionarose in an attempt to provide an improved laser apparatus for laserprocessing, so that higher processing speeds may be obtained withimproved workpiece processing results.

SUMMARY OF THE INVENTION

An object of the present invention is to provide improved methods andsystems for laser processing a workpiece and improved methods andapparatus for controlling beam quality therein.

In carrying out the above objects and other object of the presentinvention, a method for laser processing a workpiece using an opticallypumped solid state laser apparatus having a rated CW output power ofabout 100 W or more is provided. The method includes modulating anoptical pump to directly generate a modulated laser output beam havingpeak power greater than the rated CW output power. The pump iscontrollable with a time varying electrical signal so as to provideaverage output power up to the rated CW power. The method furtherincludes limiting at least one of diameter of the output beam and fullangle beam divergence of the output beam to produce a beam quality ofabout 100 mm-mrad or better. The method still further includes focusingthe output beam, and irradiating a surface of the workpiece, with thefocused output beam to modify a physical property of the workpiece.

The rated CW output power may be up to about 4 KW.

The output beam may have a frequency less than about 1.5 KHz, or mayhave a frequency in a range of about 500 Hz-1.5 KHz.

The output beam may be periodic, or may be approximately a square-waveor a sine-wave.

The peak power of the output beam may be up to about 250% of the ratedCW output power.

The processing may comprise cutting a non-metallic workpiece.

The workpiece may comprise a metal and, the processing may comprisescribing.

The laser apparatus may be pumped by a lamp, and the step of modulatingmay include applying an AC signal across the lamp.

The solid state laser apparatus may comprise a rod, and the method mayfurther comprise transmitting a beam within a resonator through anaperture positioned within the resonator. The aperture is positionedimmediately adjacent to the rod so that the beam quality is stabilizedin the presence of a thermal lens effect.

The modulated laser output beam may be in the form of an envelopecomprising first and second bursts of pulses within first and secondenvelope portions, respectively.

The step of irradiating may result in by-products which substantiallyshield the focused output beam, whereby power density at the workpieceis attenuated. The shielded output beam is reduced in a subsequent timeinterval. The method may further include preselecting at least one of amodulation frequency and a duty cycle based on a time intervalcorresponding to a reduction in output beam shielding.

The modulated output beam may comprise: (a) a high power time intervalwherein the peak power may be greater than the rated CW power, and (b) alow power time interval wherein the peak power may be less than the peakpower during the high power time interval. The step of irradiating mayirradiate the workpiece during the high power time interval and whenshielding is reduced, whereby workpiece processing is improved relativeto operation at the rated CW power.

A diameter of the focused output beam at the workpiece may be in therange of about 0.05 mm to 1 mm.

Further in carrying out the above object and other objects of thepresent invention, a system for laser processing a workpiece isprovided. The system has a solid state laser apparatus with a rated CWoutput power. The system includes a solid state resonator including atleast one solid state laser medium, at least one reflector, and at leastone output coupler. The system further includes a pump and asuper-modulating power supply for modulating the pump so that the pumppumps the at least one solid state laser medium to generate a modulatedlaser output beam having peak power greater than the rated CW outputpower. An aperture limits at least one of a diameter of the output beamand full-angle divergence of the output beam to produce a beam qualitybetter than about 100 mm-mrad. The system further includes an opticalsystem to focus the output beam and irradiate the workpiece.

The laser apparatus may include at least one YAG element and a lamp forpumping the at least one YAG element, and the power supply may be an ACpower supply.

The laser apparatus may include at least one YAG element and a diodearray for pumping the at least one YAG element, and the power supply mayinclude a diode-based pumping system.

The solid state laser medium may include a rod and an aperture forcontrolling the beam quality. The aperture is located immediatelyadjacent the rod.

The laser medium may include a solid state laser amplifier element and adiode array for pumping the laser amplifier element, and the powersupply may include a diode-based pumping system.

Yet still further in carrying out the above object and other objects ofthe present invention, a method of controlling beam quality in a highpower laser apparatus including a laser rod having a thermal lenscharacteristic and at least one of a mirror and an output coupler isprovided. The method includes positioning an aperture at a locationimmediately adjacent the rod. The aperture has a predetermined size tocontrol beam quality.

The method may further comprise controlling beam quality during a changein a focal length of the rod. The change may result from the thermallens characteristic during operation of the laser apparatus.

The aperture may be provided in a tube for the laser rod.

Still further in carrying out the above objects and other objects of thepresent invention, a laser apparatus is provided. The laser apparatusincludes a laser resonator having at least one laser rod, and a laserrod tube fitted to an end of the at least one rod. The tube may definean aperture which controls beam quality of a laser beam generated by theapparatus. The aperture may be located immediately adjacent the at leastone rod.

The aperture may be defined by an inner profile of the tube.

The apparatus may include a plurality of laser rods. The aperture iscloser to one of the rods than to an optically adjacent end of anotherrod.

A ratio of cross-sectional areas between the at least one rod and theaperture may be approximately 36/25.

The aperture may be about 3 mm from an end of the at least one rod.

Yet still further in carrying out the above object and other objects ofthe present invention, a laser rod is provided which has a tubeconnected at one end thereof. The tube is provided with an apertureimmediately adjacent an end of the rod for controlling beam quality of alaser beam generated by the rod when mounted in a pumping chamber.

Yet still further in carrying out the above object and other objects ofthe present invention, a laser apparatus is provided including a laserrod mounted in a pumping chamber, and an aperture immediately adjacentan end of the rod for controlling beam quality of a laser beam generatedby the apparatus.

The aperture may be formed by the rod, or may be formed by shaping anend of the rod.

The end of the rod may be beveled.

An internal projection may be formed in the rod to form the aperture.

A high speed method of cutting a semiconductor workpiece with a laser isprovided. The workpiece has a brittle characteristic and is susceptibleto cracking when cut with a high energy laser beam. The method includesgenerating a modulated laser output beam having a frequency in the rangeof about 500 Hz to 1500 Hz, a maximum peak output power of about 1600 Wover a focused spot having a size of about 0.05 mm to 0.3 mm, and a beamquality about 100 mm-mrad or better over the focused spot. The methodfurther includes irradiating a surface of the workpiece with the spotduring relative motion of the workpiece and the spot whereby cuttingspeed and edge quality of the cut workpiece is improved and undesirablemicrocracking of the cut workpiece is avoided.

The laser may have a rated CW power and the maximum peak output powerexceeds the rated CW power.

The workpiece may be made of a silicon material having a thickness ofabout 0.5 mm.

The peak output power may be in the range of about 200 W-400 W and thebeam quality may be about 32 mm-mrad or better.

The cutting speed may be greater than about 2 meters/minute.

A semiconductor device may be made by the cutting method.

In a high power, solid state, laser-based processing system for welding,cutting, scribing, and the like, a workpiece, a method is provided. Themethod includes generating a modulated solid state laser output beamhaving at least one of a predetermined modulation frequency in a rangeof 0 to about 1.5 KHz and duty cycle. The method also includes focusingthe laser output beam, and irradiating a surface of the workpiece withthe focused output beam to modify a physical property of the workpiece.The step of irradiating results in by-products which substantiallyshield the focused output beam, whereby power density of the focusedoutput beam at the workpiece is attenuated. The shielded beam is reducedin a subsequent time interval. The method may further includepreselecting at least one of the predetermined modulation frequency andthe duty cycle based on a time interval corresponding to a reduction ofshielding.

The modulated output beam may comprise: (a) a high power time intervalwherein the peak power is greater than a rated CW power, and (b) a lowpower time interval wherein the peak power is less than the peak powerduring the high power time interval. The step of irradiating mayirradiate the workpiece during the high power time interval and whenshielding is reduced, whereby workpiece processing is improved relativeto operation at the rated CW power.

A solid state laser system for carrying out the above method is furtherprovided.

The above system may include a super-modulating power supply.

The above object and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a laser pumping chamber;

FIG. 2 is a graph of power versus time which shows power output of aconventional CW laser when modulated up to the CW level;

FIG. 3 is a schematic view of a laser resonator;

FIG. 4 is a graph of power versus time which shows a sine wave, supermodulated output;

FIG. 5 is a graph of power versus time which shows a sine wave,modulated output of different modulation depth;

FIG. 6 is a graph of power versus time which shows a square wavemodulated output;

FIG. 7 is a schematic view of laser resonator;

FIG. 8 is a view, partially broken away and in cross section, of a laserrod with attached laser rod tube;

FIG. 9 is a view, partially broken away and in cross section, of a laserrod with laser rod tube forming an aperture;

FIGS. 10 a-10 c are side schematic views of laser resonators ofdifferent characteristics;

FIG. 11 is a graph of power input versus beam quality;

FIG. 12 is a view, partially broken away, of an alternative embodimentof the present invention;

FIG. 13 is a view, partially broken away, of a further alternativeembodiment of the present invention;

FIGS. 14 a-14 c are top schematic views (FIGS. 14 b and 14 c beingenlarged) which illustrate improvements in the cutting of silicon usinga method and system of the invention;

FIGS. 15 a and 15 b are sectional schematic views which show a laserhead used for a scribing operation; FIG. 15 a is an end sectional viewand FIG. 15 b is a sectional view along lines 15 b-15 b;

FIG. 16 is a graph of welding speed versus material thickness forvarious types of modulation;

FIG. 17 is a graph of beam penetration versus defocus for variousmodulation types; and

FIGS. 18 a-18 b are a schematic view of a system of the presentinvention and a graph of weld penetration versus air knife height,respectively, which show welding results obtained with CW and sine wavemodulated beams at different air knife positions relative to theworkpiece.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 shows a laser resonator. In one embodiment of the invention, thisresonator includes two flooded ceramic cavities 10 and 11 each housingan Nd:YAG rod 12 and 13, respectively. Alternatively, two dry goldcavities may be used with the same AC power supply. Each chamberincludes two arc lamps fitted with electrodes for AC excitation,typically up to 8 KW per lamp at 20-30 KHz. (e.g.: 15 KW total perchamber, 30 KW total lamp power).

FIG. 1, by way of example, depicts a single arc lamp 5 and an AC source6. The resonator of FIG. 3 is formed between two flat mirrors 14 and 15which are spaced 1500 mm apart to give a resonator pitch of 750 mm. Thecavities 10 and 11 are arranged to give a symmetrical periodic resonatorcontrolled by three apertures 16, 17 and 18 each having a 3.3 mmdiameter placed at the center point and close to the mirrors 14 and 15.

The resonator output is imaged down by approximately nine times using500 mm and 60 mm focal length lenses 19 and 20, respectively, afterpassing beam turning mirrors 21 and 22 and launched into a 400 mm corediameter fiber 23 of approximately 5 M in length.

Although not shown in FIG. 3, a beam produced from the apparatus iscollimated by a 160 mm achromat and focused using a 100 mm achromat,with a compensating meniscus lens, in a cutting head, to give a spotsize of 0.27 mm. In other versions of the apparatus, a 200 mm achromatrecollimating lens was refocused by a 80 mm triplet lens to give a spotsize of 0.16 mm.

The laser as described is used to generate 750 W of power at the sourcefor a drive level of 30 KW, with a beam quality of approximately 70-80mm-mrad (diameter×full-angle). The laser was tested using a power supplyallowing sine and square wave modulation at enhanced peak power (i.e.peak power greater than the CW level).

As shown in FIG. 4, a super modulation process may be used in someembodiments of the invention to enable the modulated beam to beincreased, either periodically or non-periodically, above the CW level.In FIG. 4, super modulation in the form of a sine wave is shown whichcan modulate the beam, in this case periodically, to power of up to 250%or so of the CW level. Methods of achieving modulation are, inthemselves, well known. The depth and frequency, f, of modulation may bevaried and, as shown in FIG. 5, may be varied from a large depth ofmodulation to, as shown in FIG. 4, a relatively small depth ofmodulation about the CW level. FIG. 6 shows an alternative type ofmodulation in which square wave modulation is used and, in preferredembodiments, modulation levels up to about 250% of CW level have beenused.

However, it should be noted that, in other embodiments of the invention,the output can be sine wave, square wave, or any other wave form (e.g.sawtooth wave), which may be periodic or non-periodic.

The modulation frequency of some embodiments of the invention ispreferably between 0 Hz and 1500 Hz. In this example, a high beamquality of about 70 to 80 mm-mrad in combination with super modulationcan provide the laser with peak output powers of up to about 2 KW at 100Hz modulation frequency, and this falls to about 1700 W at 1440 Hzsquare wave modulation.

The super-modulated laser apparatus of some embodiments of the inventionis controllable over a wide frequency range, preferably from DC to themaximum frequency of interest. Further, unlike conventional pulsedlasers, the super-modulated design provides for nearly flat, or at leastslowly varying available mean output power over the entire frequencyrange of operation (e.g: 0-1.5 KHz). Hence, one preferredsuper-modulated laser apparatus with controllable AC excitation of thepresent invention provides capability for operation and testing with awide variety of waveforms, including but not limited to sine-waves,square wave, ramps, and burst operation.

However, it is to be recognized that alternative pulsed laserarrangements may be used to practice certain processing methods andlaser processing applications, for instance, cutting, welding, andscribing applications later described herein. As such, the inventionshould not be considered as restricted to the preferred high power laserapparatus.

Beam Quality in High Power Lasers

Beam quality can be selected by adjustment of resonator length and useof beam-diameter apertures, as is well known, since beam quality isproportional to the area of a limiting aperture, and inverselyproportional to effective length of a resonator, as is known. Othermethods of adjusting beam quality are known to those skilled in the artas discussed earlier.

Beam quality of about 70-80 mm-mrad was produced with the arrangement ofFIG. 3, but further improvements were desirable.

Referring now to FIG. 7, a laser rod, which in a solid state laser isoften a Nd:YAG laser rod 101, is mounted in a resonator between twomirrors 102 and 103 in conventional fashion. One of the mirrors 102 and103 is half silvered so that the laser beam can be emitted from thatend. Lasing is stimulated by a pumping mechanism, which is usually inthe form of pumping lamps 104 and 105. The apparatus describedimmediately above is mounted within a housing (not shown), which keepsthe components in a predetermined relationship with respect to eachother.

As previously described, laser rod tubes are often fitted to each end ofa laser rod. FIG. 8 shows a laser rod tube 106 fitted to the laser rod101. One or more seals 107, typically of Teflon™ or similar material,may be used in affixing the tube 106 to the rod 101 to avoid ingress ofcooling water and to create a good seal. The tube 106 is then used tomount the rod 101 in the pumping chamber and to provide a means of watersealing, as is described in the applicant's co-pending patentapplication No. GB 0130971.1. In embodiments of the present invention,this rod tube 106 is used to provide an aperture for controlling beamquality and which overcomes the problems of prior art devices forcontrolling beam quality associated with centering, cooling and laserrise time.

In at least one embodiment of a super-modulated laser system of thepresent invention, an aperture may be placed close to the rod 101.

Referring now to FIG. 9, an aperture 109 of diameter significantlysmaller than the diameter of rod aperture 108 is machined into the innerprofile of a rod tube. This aperture 109 controls the beam quality ofthe laser. The aperture 109 may typically be positioned so that, in use,it is positioned about 3 mm from the end of the rod 101 but this may bevaried. Usually, the aperture 109 will not be mounted right against therod 101 but in certain circumstances it could be.

A typical laser rod has a diameter of approximately 6.35 or 6.00 mm. Thediameter D of the aperture is selected to have a selected beam quality.For a high beam quality, in one embodiment, the aperture is of diameter4.83 mm. Thus, the ratio of cross-sectional areas is approximately36/25.

As previously described, the beam quality of a laser beam is determinedby the divergence of the beam times the diameter of the beam at a waistposition. The diameter of the waist depends upon the thermal lensing ofthe system and, for a given system thermal lensing, is determined bypump power. At low pump power, thermal lensing is low and the diameterof the waist is at its largest. As pump power increases, lensingincreases and so the waist diameter decreases.

FIG. 10 a shows low pumping power, where there is accordingly littlethermal lensing. Laser energy indicated by phantom lines 110 within theresonator therefore lies Within a generally parallel envelope and doesnot significantly converge or diverge from the line of the rod 101 tothe mirrors 102 and 103. It will therefore be apparent that if anaperture 108 a (shown in dashed lines in FIGS. 10 b and 10 c) is placedanywhere between an end 109 of the rod 101 and the mirror 102 (or mirror103) then the aperture 108 a will limit the diameter of the beam andwill therefore directly affect beam quality.

FIG. 10 b shows the position in which the pumping power has beenincreased to thereby increase thermal lensing up to the confocal pointof the resonator, where the focal length f of the rod is equal to theeffective length of the resonator L/2, i.e. f=L/2. This is known as aconfocal resonator. As is described, when the laser is switched fromcold (low power) to high power the waist diameter of such a systemdecreases from the diameter generally shown at 108 b in FIG. 10 b to thediameter shown as 110 a in FIG. 10 b. Thus, an aperture placed atposition 108 a falls within the envelope of a beam within the resonatorboth at the turn on stage and subsequently, since it is within bothenvelopes 108 a and 108 b and so the aperture controls beam quality fromstart up. However, if an aperture is placed nearer to a mirror 102 or103 or output coupler than to the end 109 of the rod 101, as shown atposition 108 b, for example, or, alternatively, if placed closer to anadjacent resonator n in a multiple resonator set up, then the aperturewill affect beam quality not at lower powers but at higher powers, asthe beam converges, will not affect it since the beam envelope will thenlie wholly within the aperture within the area defined by the aperture.

With increased beam power, a situation such as that shown in FIG. 10 cresults. In this example, the focal length, f, is equal to L/4. As suchpowers, an aperture closer to the end 109 of the rod 101 than to amirror 102 or 103 (or an adjacent rod) is effective at controlling beamquality whereas one nearer to the mirror 102 or 103 is not since thebeam envelope falls wholly within the aperture and therefore is notaffected by it.

In some embodiments, the aperture lies within 0 to 5 mms, typically 3mm, of the end 109 of the rod 101. In embodiments of the invention, theaperture may lie closer to the rod 101 than to an adjacent mirror 102 or103 or the end of an adjacent rod.

FIG. 11 shows schematically a plot of power input against beam quality.A plot 120 is for a system in which no aperture is provided. Plot 121 isfor a system where an aperture is provided near the mirror 102 or 103,i.e., in a position such as 108 b in FIGS. 10 b and 10 c. A plot 122 isfor a position where an aperture is provided near the rod 101 (apertures108 a). The plots 120, 121 and 122 show how beam quality varies as theinput increases, passing the situation shown in FIGS. 10 a, 10 b and 10c at positions A, B and C, respectively, in FIG. 11. The beam qualityplot 120 for a non-apertured system is in the format as shown at B ofmaximum beam quality for a particular input power. Where an aperture isprovided near to the mirror 102 or 103 (position 108 b) then beamquality is initially good. However, it deteriorates as input increasesas the effects of the aperture reduce, as described above. With theaperture controlling beam quality, plot 122, the overall plot is againGaussian generally but is lower magnitude (i.e. better beam quality)than the non-apertured configuration.

A balance is required between beam quality and between laser outputsince laser output generally will initially increase with increasedinput but then begins to decrease.

In alternative embodiments, the aperture may be provided by the rod 101itself, rather than being provided by a laser rod tube or other externalmeans. FIG. 12 shows an embodiment in which the end of a rod is beveled123 to create an aperture. It may be otherwise shaped to create asuitable aperture.

FIG. 13 shows an alternative version in which a notch 124 or other meansis formed in a rod 123, which provides an aperture defining the width ofthe beam at that point.

In embodiments of the invention, the aperture is automatically centeredon the rod axis by mechanical tolerances, particularly if the apertureis formed in the inner profile of the rod tube or otherwise associatedwith the rod tube and therefore problems of centering are reduced oreliminated. Furthermore, the laser rod is generally water-cooled and theaperture itself is cooled by the water flow cooling the laser rod. Inaddition, since the aperture is placed close to the end of the laserrod, the transmission of the aperture is insensitive to laser pumppower/rod thermal lensing as described and therefore does not limit thelaser rise time as an aperture placed close to a mirror might.

It is to be recognized that the beam quality improvement obtained is notrestricted in its application to preferred super-modulated systems butmay be utilized in numerous solid state laser systems where the beamquality is affect by thermal lensing. For example, the improvement maybe used with high power, diode pumped, CW lasers.

Additional Alternatives with Super-Modulation and High Beam Quality

In at least one embodiment a commercially available JK-Model laserproduced by the assignee may be used for laser processing in accordancewith the present invention. Specific JK models have rated average outputpowers of 200, 400, 500, 800, and 1000 W. The 800 W and 1000 W versions,the JK models 802 and 1002 respectively, also include a periodicresonator arrangement with two rods in series (similar to FIG. 3). Themodels incorporate the preferred super-modulation approach and enhancedbeam quality with placement of an aperture close to the output of therod(s). These models may be used to produce a high power laser beam soas to cut, weld, or otherwise modify a workpiece. Preferably the laserwill be fiber coupled to a remote head which in turn will be manipulatedby a robot, for instance.

By way of example, FIG. 15 b schematically shows a laser coupled by anoptical fiber 23 to a cutting head 65 for a laser scribing operation (tobe discussed later). Further details of exemplary robotic laser headsand applications can be found in patent publication WO 0054925 and U.S.Pat. No. 5,428,280.

Exemplary Laser Applications

Laser Cutting

Trials have been carried out using the laser described in FIG. 3, usingboth conventional CW and also super modulated beams, at power levels upto 700 W and with 0.16 mm and 0.27 mm spot sizes. Cutting was performedwith air assist at 4 bar pressure. Best results were obtained with themodulated beam at maximum repetition rate (1440 Hz, 640 W average powerand 1610 W peak power). In another embodiment, exemplary parametersincluded a frequency in the range of about 500-1500 Hz, approximate 200W average power and 400 W peak powers, with about 32 mm-mrad, beamquality over a focused spot diameter in a range of about 0.08-0.3 mm.

Referring now to FIGS. 14 a-14 c, the apparatus was used for cuttingpolycrystalline silicon material, which is conventionally manufacturedfor solar cells. Substrate 40 is an example. The material was about 0.5mm thick. This material is generally extremely brittle and prone tocracking during laser cutting. It is generally cut with conventionallasers at speeds of around 1.5 M/minute. A path 42 shows a typical,conventional cutting path, whereas a path 41 is a path made with acutting head using an embodiment of the present invention. Edge qualitywith conventional laser processing is generally acceptable butmicro-cracking 43 is often produced and this can lead to failure of somecomponents during subsequent process steps and reduction in yield. Whena conventional laser apparatus with poorer beam quality was used with aconventional pulsed beam, then poor edge quality 42 and unacceptablecracking 43 were found. However, using a modulated scheme of the presentinvention, cutting speeds of up 14 M/minute were achieved and asignificant improvement in edge quality 44 and reduction in cracking 45were noted.

Other advantages of modulation include better cutting quality and speedand other advantages such as reduction of uncontrolled burning whilstretaining process speed and stability, improvement in off-normalcutting. Cutting at an angle to the surface and cutting around a bend intwo or more axes where the speed of the motion device can be high butthe speed of the laser focus on the surface of the part can be slow canresult in overburn of the cut or an intermittent cut. The material to becut can be overheated by the slower speed in the bend and/or theincreased relative thickness of the material to be cut. This can resultin extra burning of the base metal or a cut that does not completelypenetrate. Extra burning will create a very large kerf width that can bemore than 20 times the kerf width that can cause a scrap part. Supermodulation at the same mean power or at a lower mean power can cutthrough the relatively thicker material without the high mean powersthat can cause overburning.

Also, at lower mean power but maintaining the peak power will result ina more consistent cut around a corner without overburn. Better cuttingof many materials, for example, galvanized steels and many othermaterials including aluminum alloys, is achieved. In addition,modulation is particularly beneficial for piercing materials. If a slugis to be cut out of a material then the normal way this is done is byfirst piercing the material and then making a closed cut which causesthe slug to drop. The use of modulation and thereby higher peak powerimproves the piercing and improves the cutting process. Smaller, highaspect ratio, piercing holes may be made using modulation in combinationwith high beam quality.

Preferably, a method and system of the invention having beam qualitybetter than about 100 mm-mrad (full angle measurement) is used (that is,numerical values less than or equal to about this) together with the useof super modulation, i.e. modulation in which the beam is, whetherperiodically or non-periodically, modulated above the CW level. Thisleads to lasers of lower average power but high peak power. Typically,modulation frequencies between 0 Hz and 1500 Hz may be used, withvarying duty cycle.

Laser Scribing

Another application where super modulation creates improvements over CWand pulsed lasers is in scribing components. A scribe is a kerf cut intoa material that does not penetrate through to the back side. Other termssuch as a blind cut or blind slot could also be used. FIGS. 15 a-15 bshow a standard scribe process where a laser beam 64 coming from afocusing assembly 65 that might also incorporate a coaxial gas nozzledelivers the laser beam onto the workpiece or target part 63 creating ascribe 61 to a depth 62.

Metal and non-metal parts are scribed for several reasons. For example,the scribe could be a chip-breaking initiator in a subsequent machiningprocess, marking, blind kerf cutting for subsequent part detachment asin cutting nearly through a weld, and for blind slot cutting where thefeature is required in the part. CW lasers produce a more uncontrolledburn, especially at the bottom of the scribe that can become a largecircular well at the bottom of the scribe. CW lasers do not have theextra scribe depth capability of a supermodulated laser. CW lasers runthe risk of explosive oxygen overburn with some materials and the debrisbeing removed can interfere with the incoming laser beam or clog thescribe for inconsistent scribe depth with more debris in or near thescribe. Pulsed lasers with high peak powers can scribe somewhat deeperthan a supermodulated laser. However, their relatively slow pulsing rateand lower average power at high pulsing frequencies reduce theirscribing speed and/or depth at high speed compared to a supermodulatedlaser.

Pulsing or using a super modulated laser can benefit the laser scribeprocess in several ways. It improves the coupling of the laser energyinto the material. It increases the amount of controlled burning andvaporization/expulsion of material and can make a deeper scribe. Itincreases the effective depth of focus for deeper and more consistentscribes, and the “off time” allows time for the spatter to be ejectedimproving depth and quality. Super modulation at high frequencies allowsfaster scribing and/or deeper scribing at high speeds.

Laser Welding

The plots 30-32 of FIG. 16 show an example of how welding speed improvesfor a laser of high beam quality and modulation, as opposed to CWoperation. In the figure, three sets of data of the plots 30, 31 and 32are provided, each showing how welding speed varies with materialthickness for a particular type of modulation. The first plot 30 showsthe situation when CW is used. Plot 30 shows welding speed (in metersper minute) against material thickness in millimeters, for a CW output.Plot 31 shows this for a sine wave output, and plot 32 shows this for asquare wave super modulated output. The material to be welded was 304stainless steel. It is seen that at a material thickness of 1 mm, thereis a 40% increase in speed between the CW and square wave performance.At 3 mm, the increase is very substantial, about 150%.

The plots of FIG. 17 shows the effect of processing depth of focus(e.g.: as used herein refers to the depth range over which acceptablemachining occurs without changing the relative position of the workpieceand beam focal point) compared between CW and square wave modulation.The CW output of the laser was 470 W and for a square wave modulatedoutput, at a modulation frequency of 180 Hz, the average power was 350 Wand peak power of 700 W. The spot size is 0.30 mm and the work piece isstainless steel. It is seen that a significant improvement is madebetween the CW and square wave outputs.

As FIG. 17 shows, the depth of focus with square-wave modulation isimproved. In the example shown, it is about 0.66 mm (d₂) whereas thatfor CW is about 0.58 mm (d₁). Additionally, the depth of focus for thesame penetration (d₃) (i.e. where the plots for CW and for square-waveachieve the same penetration) is improved from 0.58 to about 0.80, i.e.about 138%.

Approximately 10% more penetration is achieved using the 350 W modulatedoutput rather than the 480 W CW one. Since the average power is less,this is more economical and, in practice, uses about 16% lesselectricity to create the same weld. This can lead to significantadvantages such as significantly longer lamp life (typically 25%longer). Also, the depth of focus is improved by nearly 40% for the sameweld penetration.

Thus, by using high peak power modulation, a laser of lower averagepower can weld to a greater penetration than a similar CW unit but witha reduced heat input. As a practical example of this, when welding acomponent such a fuel injector where high heat input can distort veryfine parts, welding with a sine-wave output produces the samepenetration as CW output but with no distortion. Furthermore, high peakpower improves the processing of reflective materials and materials withhigh heat conductivity, such as aluminum alloys.

Unlike CO2 lasers, Nd:YAG laser light is not absorbed in the weld plumeby a reverse-bremsstrahlung mechanism. Using helium assist gas to coolthe weld, plume can minimize the absorption with CO2 lasers. Tests haveshown that using a high heat conductivity helium shield gas does notimprove the weld nugget profile as it does with CO2 lasers so themechanism appears to be different. The scattering of the YAG laser lightis more prevalent due to its shorter wavelength and the scattering ofthe laser light due to soot and other small particles in the weld plumecould explain the widening weld nugget and loss of penetration seen withCW output as compared to super modulated.

FIG. 18 a shows a laser welding head 55 with an air knife at variablepositions 51 and 53 above a weld area 56. Using an air knife at theclose position 51 pushes the weld plume and soot away from the incominglaser beam 52 and results in significantly more penetration with CWoutput than without an air knife or with the air knife located at theposition 53 further above the weld plume.

When super modulation was used the penetration remained nearly constantfor any air knife position above the weld area 56. The plots of FIG. 18b shows this result. The plots demonstrate that the soot cloud above theweld takes some time to form and to reach a concentration thatsignificantly scatters the laser beam. If modulation is used to allowsome time for the particulate cloud to dissipate during the low powercycle, then the next high power cycle in the modulated output will notbe scattered to produce better welding.

Some materials appear to create more scattering soot than others andappear to have a maximum modulation frequency limit above whichinsufficient soot dissipation occurs and weld penetration issignificantly reduced and weld width can increase. Some materialsbenefit from modulation frequencies of no more than about 300 Hz andothers appear to benefit at frequencies above about 1 kHz.

Penetration level could also play a role in the maximum frequency. Useof high speed cross-flow air knives is not beneficial in many weldingapplications since it makes shielding of the weld area with inert gasdifficult and costly due to the volume flow rate of gas and entrainmentof oxygen from the air from high speed turbulent flow. Also, a greatdeal more of the costly inert gas is required as well as the gas flow'stendency to push and froth the molten weld metal creating a rough weldsurface, undercutting, and porosity, etc.

Summary of Some General Aspects

Accordingly, lower average power lasers using the modulation techniquesof the present invention process thicker materials faster than a CWlaser of possibly higher average power.

Note that in the embodiments, the modulation depth and duty cycle can bealtered.

The present invention includes any modulation technique that modulates,either periodically or not, a beam above the CW level. The modulationmay be by use of power supply techniques, i.e. simply increasing thepower supplied to a pumping lamp above that of a level which reduces theCW output or by pumping methods or by other means. Methods of modulatinglaser outputs are, in themselves, known, as are methods of improvingbeam quality.

The invention is applicable to any solid state laser and most preferablyto Nd:YAG lasers.

The invention is generally applicable to lasers having a beam qualitybetter than 100 mm-mrad.

The beam quality may be improved by placing an aperture closer to therod than to a mirror or output coupler. Alternatively, the rod may beshaped to create a suitable aperture as described herein.

Parameters such as angle of cut and quality of cut are significantlyimproved using the present invention. High quality welding results maybe obtained by controlling the frequency and duty cycle of modulation.

A laser based method of processing a workpiece using an optically pumpedsolid state laser apparatus generally comprises: (1) generating amodulated laser output beam having a modulation frequency and beamquality less than about 100 mm-mrad, the beam quality defined by theproduct of beam diameter-full angle; and (2) irradiating the workpiecewith the laser output beam.

While the best mode for carrying out the invention has been described indetail, those familiar with the art to which this invention relates willrecognize various alternative designs and embodiments for practicing theinvention. For instance, though the lamp pumped solid state laser systemand AC driver are regarded as preferred for carrying out the invention,a diode pumped system may be modulated above the average power level toweld, cut, or otherwise modify the workpiece with the pulse parametersdisclosed herein.

Similarly, one or more optical amplifiers having the desired peak power,pulse width, and pulse temporal spacing may be used for certainworkpiece processing applications.

Where suitable, other solid state laser elements such as a fiber orwaveguide device may replace the Nd:YAG rod. It is thereforecontemplated that the following claims shall cover any suchmodifications or variations of the described embodiments as fallingwithin the scope of the invention.

1. A method for laser processing a workpiece using an optically pumpedsolid state laser apparatus having a rated CW output power of about 100W or more, the method comprising: modulating an optical pump to directlygenerate a modulated laser output beam having peak power greater thanthe rated CW output power, the pump being controllable with a timevarying electrical signal so as to provide average output power up tothe rated CW power; limiting at least one of diameter of the output beamand full angle beam divergence of the output beam to produce a beamquality of about 100 mm-mrad or better; focusing the output beam; andirradiating a surface of the workpiece with the focused output beam tomodify a physical property of the workpiece.
 2. The method as claimed inclaim 1, wherein the rated CW output power is up to about 4 KW.
 3. Themethod as claimed in claim 1, wherein the output beam has a frequencyless than about 1.5 KHz.
 4. The method as claimed in claim 1, whereinthe output beam has a frequency in a range of about 500 Hz-1.5 KHz. 5.The method as claimed in claim 1, wherein the output beam is periodic.6. The method as claimed in claim 1, wherein the output beam isapproximately a square-wave or a sine-wave.
 7. The method as claimed inclaim 1, wherein the peak power of the output beam is up to about 250%of the rated CW output power.
 8. The method as claimed in claim 1,wherein the processing comprises cutting a non-metallic workpiece. 9.The method as claimed in claim 1, wherein the workpiece comprises ametal and wherein the processing comprises scribing.
 10. The method asclaimed in claim 1, wherein the laser apparatus is pumped by a lamp, andwherein the step of modulating includes applying an AC signal across thelamp.
 11. The method as claimed in claim 1, wherein the solid statelaser apparatus comprises a rod, and wherein the method furthercomprises transmitting a beam within a resonator through an aperturepositioned within the resonator, the aperture being positionedimmediately adjacent to the rod so that the beam quality is stabilizedin the presence of a thermal lens effect.
 12. The method as claimed inclaim 1, wherein the modulated laser output beam is in the form of anenvelope comprising first and second bursts of pulses within first andsecond envelope portions, respectively.
 13. The method as claimed inclaim 1, wherein the step of irradiating results in by-products whichsubstantially shield the focused output beam, whereby power density atthe workpiece is attenuated, the shielded output beam being reduced in asubsequent time interval, the method further comprising: preselecting atleast one of a modulation frequency and a duty cycle based on a timeinterval corresponding to a reduction in output beam shielding.
 14. Themethod as claimed in claim 13, wherein the modulated output beamcomprises: (a) a high power time interval wherein the peak power isgreater than the rated CW power, and (b) a low power time intervalwherein the peak power is less than the peak power during the high powertime interval, wherein the step of irradiating irradiates the workpieceduring the high power time interval and when shielding is reduced,whereby workpiece processing is improved relative to operation at therated CW power.
 15. The method as claimed in claim 1, wherein a diameterof the focused output beam at the workpiece is in the range of about0.05 mm to 1 mm.
 16. A system for laser processing a workpiece, thesystem having a solid state laser apparatus with a rated CW outputpower, the system comprising: a solid state resonator comprising atleast one solid state laser medium, at least one reflector, and at leastone output coupler; a pump and a super-modulating power supply formodulating the pump so that the pump pumps the at least one solid statelaser medium to generate a modulated laser output beam having peak powergreater than the rated CW output power; an aperture for limiting atleast one of a diameter of the output beam and full-angle divergence ofthe output beam to produce a beam quality better than about 100 mm-mrad;and an optical system to focus the output beam and irradiate theworkpiece.
 17. The system of claim 16, wherein the laser apparatuscomprises at least one YAG element and a lamp for pumping the at leastone YAG element, and wherein the power supply is an AC power supply. 18.The system of claim 16, wherein the laser apparatus comprises at leastone YAG element and a diode array for pumping the at least one YAGelement, and wherein the power supply comprises a diode-based pumpingsystem.
 19. The system of claim 16 wherein the solid state laser mediumcomprises a rod and an aperture for controlling the beam quality, theaperture being located immediately adjacent the rod.
 20. The system ofclaim 16, wherein the laser medium comprises a solid state laseramplifier element and a diode array for pumping the laser amplifierelement and wherein the power supply comprises a diode-based pumpingsystem.
 21. A method of controlling beam quality in a high power laserapparatus including a laser rod having a thermal lens characteristic andat least one of a mirror and an output coupler, the method comprising:positioning an aperture at a location immediately adjacent the rod, theaperture having a predetermined size to control beam quality.
 22. Themethod as claimed in claim 21 further comprising controlling beamquality during a change in a focal length of the rod, the changeresulting from the thermal lens characteristic during operation of thelaser apparatus.
 23. The method as claimed in claim 21, wherein theaperture is provided in a tube for the laser rod.
 24. A laser apparatuscomprising: a laser resonator including at least one laser rod; and alaser rod tube fitted to an end of the at least one rod wherein the tubedefines an aperture which controls beam quality of a laser beamgenerated by the apparatus and wherein the aperture is locatedimmediately adjacent the at least one rod.
 25. The apparatus as claimedin claim 24, wherein the aperture is defined by an inner profile of thetube.
 26. The apparatus as claimed in claim 24, comprising a pluralityof laser rods, the aperture being closer to one of the rods than to anoptically adjacent end of another rod.
 27. The apparatus as claimed inclaim 24, wherein a ratio of cross-sectional areas between the at leastone rod and the aperture is approximately 36/25.
 28. The apparatus asclaimed in claim 24, wherein the aperture is about 3 mm from an end ofthe at least one rod.
 29. A laser rod having a tube connected at one endthereof, the tube being provided with an aperture immediately adjacentan end of the rod for controlling beam quality of a laser beam generatedby the rod when mounted in a pumping chamber.
 30. A laser apparatuscomprising: a laser rod mounted in a pumping chamber; and an apertureimmediately adjacent an end of the rod for controlling beam quality of alaser beam generated by the apparatus.
 31. The laser apparatus asclaimed in claim 30, wherein the aperture is formed by the rod.
 32. Thelaser apparatus as claimed in claim 30, wherein the aperture is formedby shaping an end of the rod.
 33. The laser apparatus as claimed inclaim 30, wherein the end of the rod is beveled.
 34. The laser apparatusas claimed in claim 30, wherein an internal projection is formed in therod to form the aperture.
 35. A high speed method of cutting asemiconductor workpiece with a laser, the workpiece having a brittlecharacteristic and being susceptible to cracking when cut with a highenergy laser beam, the method comprising: generating a modulated laseroutput beam having a frequency in the range of about 500 Hz to 1500 Hz,a maximum peak output power of about 1600 W over a focused spot having asize of about 0.05 mm to 0.3 mm, and a beam quality about 100 mm-mrad orbetter over the focused spot; and irradiating a surface of the workpiecewith the spot during relative motion of the workpiece and the spotwhereby cutting speed and edge quality of the cut workpiece is improvedand undesirable microcracking of the cut workpiece is avoided.
 36. Themethod of claim 35 wherein the laser has a rated CW power and themaximum peak output power exceeds the rated CW power.
 37. The method ofclaim 35 wherein the workpiece is made of a silicon material having athickness of about 0.5 mm.
 38. The method of claim 35 wherein the peakoutput power is in the range of about 200 W-400 W and the beam qualityis about 32 mm-mrad or better.
 39. The method of claim 35 wherein thecutting speed is greater than about 2 meters/minute.
 40. A semiconductordevice made by the cutting method of claim
 35. 41. In a high power,solid state, laser-based processing system for welding, cutting,scribing, and the like, a workpiece, the method comprising: generating amodulated solid state laser output beam having at least one of apredetermined modulation frequency in a range of 0 to about 1.5 KHz andduty cycle; focusing the laser output beam; irradiating a surface of theworkpiece with the focused output beam to modify a physical property ofthe workpiece, wherein the step of irradiating results in by-productswhich substantially shield the focused output beam, whereby powerdensity of the focused output beam at the workpiece is attenuated,wherein the shielded beam is reduced in a subsequent time interval andwherein the method further comprises preselecting at least one of thepredetermined modulation frequency and the duty cycle based on a timeinterval corresponding to a reduction of shielding.
 42. The method ofclaim 41 wherein the modulated output beam comprises: (a) a high powertime interval wherein the peak power is greater than a rated CW power,and (b) a low power time interval wherein the peak power is less thanthe peak power during the high power time interval, and wherein the stepof irradiating irradiates the workpiece during the high power timeinterval and when shielding is reduced, whereby workpiece processing isimproved relative to operation at the rated CW power.
 43. A solid statelaser system for carrying out the method of claim
 35. 44. The system ofclaim 43 wherein the system includes a super-modulating power supply.45. A solid state laser system for carrying out the method of claim 41.46. The system of claim 45 wherein the system includes asuper-modulating power supply.